The present invention relates to a semiconductor device, a semiconductor laser device, a manufacturing method for the semiconductor device, a manufacturing method for the semiconductor laser device, an optical disk device and an optical transmission system, and more particularly relates to a semiconductor laser device for use in an optical disk device and an optical transmission system and a manufacturing method therefor.
Conventionally, the buried ridge type of semiconductor laser device is in heavy usage since it has a structure having both reliability and properties. The buried ridge-type semiconductor laser device requires a plurality of crystal growth processes. Such crystal growth processes have been a large obstacle in reduction of manufacturing cost of the semiconductor laser device. This leads to development of an element production method where the buried-type crystal growth is omitted and the semiconductor laser device is produced in one crystal growth. In particular, there has been developed an element where a ridge waveguide structure is formed, and current narrowing and optical confinement are performed by using silicon oxide (SiO2) and silicon nitride (SiNx). An example of the conventional manufacturing method for the ridge waveguide semiconductor laser device is shown in FIG. 6A to FIG. 6E (see JP 62-23191 A for example).
In FIG. 6A to FIG. 6E, there are shown an n-type GaAs substrate 401, an n-type GaAs buffer layer 402, an n-type Al0.43Ga0.57As lower cladding layer 403, an undoped n-Al0.11Ga0.89As active layer 404, a p-type Al0.43Ga0.57As first upper cladding layer 405, a p-type Al0.25Ga0.75As second upper cladding layer 406, a contact layer 407, a resist film 408, an SiO2 insulating film 409, a p-side electrode 410 and an n-side electrode 411.
Description is hereinbelow given of a manufacturing method for the semiconductor laser device. First, as shown in FIG. 6A, the buffer layer 402, the lower cladding layer 403, the active layer 404, the first upper cladding layer 405, the second upper cladding layer 406 and the contact layer 407 are each grown in sequence on the n-type GaAs substrate by using any one of crystal growth methods including a liquid phase growth method, a vapor growth method and a molecular beam epitaxy (MBE) method. Next, as shown in FIG. 6B, the resist film 408 is formed into a stripe shape by photolithographic technique. Next, a ridge structure is formed by using an etchant which selectively etches only the second upper cladding layer 406. Such an etchant may include an NH3/H2O2 solution. A solution of NH4OH:H2O2=20:1 has composition dependence on the etching rate of AlyGa1-yAs, as shown in FIG. 7. In FIG. 7, the horizontal axis represents a value of y in AlyGa1-yAs, while the vertical axis represents an etching rate (μm/min)
Therefore, selective etching can be easily performed by using this etchant, so that a ridge portion is formed as shown in FIG. 6C. Next, as shown in FIG. 6D, the insulating film 409 is formed in the state that the resist film 408 remains. Then, the insulating film 409 on the ridge portion is removed together with the resist film 408 by lift-off process. Thereafter, the p-side electrode 410 and the n-side electrode 411 are formed on the upper face of the contact layer 407 and the lower face of the n-type GaAs substrate 401 respectively, by evaporation for ohmic contact. Through these steps, a ridge waveguide semiconductor laser device shown in FIG. 6E is obtained.
According to the above-stated conventional manufacturing method for the semiconductor laser device, without execution of a plurality of the crystal growth processes, there can be manufactured a ridge waveguide semiconductor laser device with an insulating film formed on an upper portion excluding the upper face of the ridge portion.
However, this conventional ridge waveguide semiconductor laser device using the above-stated insulating film had a following problem. Specifically, most of the p-side electrode on the side of the ridge portion is formed on the insulating film. An electrically conductive material constituting the p-side electrode is generally poor in adhesion to the insulating film. Therefore, there is the problem that, when a thick electrode is formed, stress thereof tends to detach the electrode from the insulating film along the interface between the insulating film and the electrode made of the conductive material. In the case of the above-described structure of the ridge waveguide semiconductor laser device, it is necessary to form the conductive material above a certain film thickness in order to prevent the conductive material from suffering step-like breakage due to a step portion of the ridge portion. Therefore, the problem of stress caused by thick material is unavoidable. Further, there is also a possibility that the electrode made of the conductive material may be detached from the insulating film along the interface therebetween in the process of bonding a metal wire to the p-side electrode for feeding current to the semiconductor laser device. This is because adhesion is weak between the conductive material of the electrode and the insulating film when ultrasonic wave or heat is generally applied to fix the metal wire to the electrode during the bonding process. If intensity of ultrasonic wave or heating is restrained in order to prevent the detachment, there is arisen a new problem of decrease in fixing strength (i.e. bonding strength) of the metal wire.